1. Field of the Invention
The present invention relates to an antenna that is used for a radio communication device or the like.
2. Description of Related Art
In recent years, small radio communication devices such as a cellular phone have been provided with various additional functions, and therefore frequency bands necessary for reception have been increased. Lately, a small radio communication device having a function of receiving a TV broadcast has become commercially practical, and such a small radio communication device needs to receive a TV broadcast frequency band.
In general, a length of an antenna corresponds to a wavelength of a signal to be received. The longer a wavelength (i.e., the lower a frequency) of a signal to be received is, the longer a length of the antenna is. UHF channels of TV broadcast use a frequency band of 470-770 MHz, while VHF channels of TV broadcast use a frequency band of 90-222 MHz. A domestic TV antenna that is installed on a roof includes radiating elements each of which has a length corresponding to a half wavelength (λ/2) of a TV broadcast signals. For example, a half wavelength (λ/2) of a TV broadcast signal of 500 MHz equal to 300 mm.
On the other hand, the small radio communication devices such as a cellular phone have become smaller. As a size of the small radio communication device is much smaller than a half wavelength (λ/2) of a TV broadcast signal, it is difficult to provide the small radio communication device with an antenna including radiating elements having lengths corresponding to a half wavelength (λ/2) of TV broadcast signals. In addition, it is desired to adopt a built-in antenna for improving a good appearance and portability of the small radio communication device. However, if a built-in antenna is adopted, a space available for the antenna will be very small.
Although a rod antenna is used for the conventional portable TV receivers, it is not suitable for a small radio communication device because of its long antenna length.
One type of the antenna that can solve the above-mentioned problem is disclosed in JP-A-2005-45599. This antenna has a wide band structure in which a zigzag conductor pattern is formed on a printed circuit board and is provided with a switch for connecting and disconnecting the conductor pattern. An antenna length is changed when the switch connects or disconnects the conductor pattern so that both the VHF band and the UVF band of TV broadcast can be received.
A usual printed circuit board has a relative dielectric constant of approximately 4.5, and the above-mentioned conductor pattern formed on the printed circuit board cannot produce sufficient contraction quantity of wavelength of a TV broadcast signal due to the relative dielectric constant of the printed circuit board, considering the frequency of the TV broadcast signal. Therefore, it is difficult to realize a substantially compact size by using the antenna disclosed in JP-A-2005-45599. In addition, the antenna disclosed in JP-A-2005-45599 includes a parasitic radiating element arranged on one side or each side of a feed conductor that is made up of the conductor pattern so that a resonance frequency is shifted to the low frequency side. However, this arrangement of the parasitic radiating element narrows a reception band. For this reason, the antenna disclosed in JP-A-2005-45599 is provided with a switch for connecting and disconnecting the conductor pattern. The switch connects or disconnects the conductor pattern so that the antenna length is changed and the reception frequency is also changed. However, if the number of the switches is small (for example, if only one switch is provided as shown in FIG. 10 of JP-A-2005-45599), it is difficult to cover a narrow reception band. In addition, it is not practical to provide a lot of switches from the viewpoint of cost or the like.